1. Field of the Invention
The present invention relates to an internal antenna for a wireless handportable communications device and to a mobile handportable containing an internal antenna.
2. Description of the Prior Art
Antennas known to be used in mobile stations for the transmission and reception of radio-frequency signals include the monopole antenna and the helix antenna. In order to guarantee correct electrical operation these antennas have to be located in free space outside the case of the mobile station. The radio-frequency signals between the radio part of the mobile station and the antenna are usually transmitted by means of conductors and connectors.
The monopole antenna is in principle a straight conductor above and substantially perpendicular to a conductive plane and its length depends among other things on the frequency range of the radio-frequency signal used. GSNI mobile communications networks, for example, use the 900-MHz frequency range, in which case the corresponding wavelengths in the air are in the range of 30 cm, approximately. Then the length of the antenna wire, which typically is about .lambda./2 for a monopole antenna, should be about 15 cm. In practical implementations the length of the antenna conductor can be shortened to some extent by using a so-called lengthening coil in the matching elements of the antenna. A known prior-art implementation of the monopole antenna in a mobile station is -such that the antenna conductor is placed inside a flexible or rigid and protective tubular piece of insulating material which is further attached to the mobile station by means of a connector. This connector also provides an electrical coupling between the antenna and the radio part of the mobile station. However, such an antenna is long in comparison to the mobile station itself and, placed outside the mobile station, susceptible to being damaged, for example by an impact upon dropping.
Another prior-art implementation of the monopole antenna is a pull-out structure such that the antenna conductor or the like placed inside the body of a mobile station can be pulled out from the mobile station e.g. for the duration of a call. When pulled out to maximum extension the antenna conductor is locked and at the same time electrically coupled to the radio part of the mobile station. After the call the antenna conductor is usually pushed back inside the case of the mobile station. However, when the antenna conductor is fully inside the case of the mobile station the signal amplification is not as good as it would otherwise be, which especially in a weak signal field may cause the connection between the mobile communications system and the mobile station to be cut off. Then the antenna has to be pulled out from the case of the mobile station if the mobile station is to be logged on in the mobile communications system, e.g. to receive incoming calls.
However, such a movable antenna comprises parts that become mechanically worn, so in the course of time there may arise a need to replace at least part of the antenna elements. Likewise, careless handling of the mobile station may damage the antenna conductor especially when being pulled out or pushed in. The wear and tear of parts and incorrect handling of the antenna may also cause, in addition to the need to replace parts, deterioration of the reliability of the mobile station.
Helix antenna is a conductor wound into a right-hand or left-hand coil placed above a conductive plane. The helix structure provides a shorter antenna than the monopole structure when the operating frequency range is the same. A known prior-art implementation of the helix antenna in mobile stations is such that the antenna conductor is placed inside a protective cylindrical or conical piece of insulating material which is further attached to the mobile station by means of a connector. This connector also provides an electrical coupling between the antenna and the radio part of the mobile station. Also this structure, being placed outside the mobile station, is susceptible to damage caused e.g. by dropping the mobile station, as well as to other external impacts.
Since an external antenna in a mobile station is susceptible to damage, the antenna itself and the attachment of the antenna to the mobile station have to be made durable and strong, which may impede the attempt to achieve the optimum electrical characteristics for the mobile station and, above all, increase the price of the antenna. As regards to the assembly work of the mobile station, an external antenna structure means more work stages and, therefore, increases the cost of the assembly work. The antenna also has to be suitable for mass production, which means that the connections between the parts of the mobile station and the different parts of the antenna have to be suitable for automatic assembly.
Particularly with small hand-held phones the external antenna may be situated near the user's head when he is using the phone, which affects the electrical operation of the antenna. Likewise, part of the electrical energy radiated by the antenna may be coupled to the user's head. To reduce that coupled power the external antenna has to be placed in such a manner that it is as far away as possible from the user's head when the mobile station is in the operating position. This also results in some limitations to the design of the mobile station.
An antenna known to avoid some of the disadvantages of the external antenna is the air-insulated planar inverted F antenna (abbr. PIFA).
The PIFA antenna comprises a thin parallelogram-shaped antenna plate made of a conductive material such as metal or a plate coated with a conductive material. The lengths of the sides of the PIFA antenna depend on the operating frequency range of the antenna. When the operating frequency range is about 880 to 960 MHz, suitable dimensions of the PIFA plate are about 50 mm for the long sides and 25 mm for the short sides. The dimensions of the antenna plate are proportional to the wavelength of the operating frequency range. The usable frequency band can also be made wider with a PIFA antenna than with a helix antenna. Then it is also easier to manufacture the PIFA antenna such that the manufacturing tolerances are sufficient to guarantee the desired electrical operation.
In the PIFA, the antenna plate is placed substantially parallel to and at a distance from the structure serving as the earth plane. The earth plane may be e.g. an at least partly conductive protective body inside the mobile station. One short side of the antenna plate is short-circuited to the earth plane by means of a conductor, and a radio-frequency signal is fed to the antenna plate via an electric circuit having a capacitance and inductance connected in series. The capacitance and inductance constitute a series-resonant circuit the resonating frequency of which is dimensioned according to the operating frequency range of the mobile station, which is about 880 to 960 MHz, for example. The signal feed point on the antenna plate may be located at the short side of the plate, near the short circuit. The capacitance in the series-resonant circuit is located e.g. in the radio part of the mobile station and the inductance is coupled between the antenna plate and the radio part.
The frequency band of the antenna also depends on the distance between the earth plane and the different elements of the antenna plate, so that if the antenna plate is moved, the frequency band of the antenna is shifted aside from the desired frequency range. In order to prevent this, the antenna plate has to be securely attached to a base. In the assembly phase of the mobile station the antenna plate has to be attached precisely at the correct distance from the earth plane. The attachment shall prevent the antenna plate from moving with respect to the earth plane both along the plane of the antenna plate and along the direction perpendicular to that plane.